The present invention relates to the manufacture of heat-insulated tubing.
German printed patent application No. 1,960,932 suggests a method for thermally insulating metal tubing, preferably copper tubing, in which the tubing is continuously fed to a machine which will provide the thermal insulation. In a first operating step, a spacer made of paper is wrapped helically around the tubing and subsequently a ribbon is wrapped around the spacer. The ribbon, in particular, is longitudinally paid and folded around the spacer. This ribbon or tape is closed to form a closed outer tube. In the space between the inner and the outer tubing, polyurethane is injected and caused to foam which is, subsequently, cured. The foam will fill the space between the inner and outer tubing completely. It can, thus, be seen that in this particular method the outer tube constitutes a kind of mold for the foam jacket thus produced. The tape or ribbon which is formed into an outer tube is preferably made from rigid polyvinyl chloride or it is made of metal. In either case, it is desired to have this outer tube provided in a rather resisting configuration. Accordingly, this particular method has been practised for the making of rigid, i.e., nonflexible, thermally insulated tubing.
The problem existed to adapt the aforementioned method of making thermally insulated tubing so that the resulting tube is flexible. Accordingly, German printed patent application No. 2,141,475 proposed a method in which the outer tubing receives an extruded copolymer layer after the foam has been cured. Due to its adhesion to the outer tubing, the copolymer layer serves as a support for that particular outer tubing, which therefore does not have to be resistive. Rather, the outer tubing may now permit bending and the copolymer layer protects the outer tubing so that it will not tear. While technically quite feasible, this particular method now had proven to be uneconomical; the resulting tubing is too expensive.
Belgian Pat. No. 670948 discloses another method for the continuous production and manufacture of a thermally insulated tubing wherein the insulation is comprised of a foam layer. In this particular method, a metallic inner tube is provided and a split tube is formed around that inner tubing, but at a spacing therefrom and under utilization of a strip. As long as the split tube is still open, a self-foaming synthetic material can be injected which will develop foam of suitable configuration. Again, the particular split tube serves as a mold; but after the foam has cured the strip material, i.e., the split tube is removed. Therefore, this particular insulated tubing is not provided with a particular outer jacket above the foam layer. Thus, the latter layer can easily be damaged. This is particularly the case if the tubing is hauled to construction sites, stored thereat, and used under the rather rough conditions that may prevail in such an area. Moreover the foam layer may be soaked through with moisture and that, of course, reduces the insulation capability. This is particularly noticeable if the tubes are installed in still-moist rooms, under moist plaster or the like. Another disadvantage of that particular method and of the tubing made in accordance with the method is that the surface is rather rough. The surface roughness is derived from excessive deforming of the strip which must be quite strong and rather thick; since it is also necessary that the strip while serving as a mold is, in effect, sealed in order to avoid escape of foam, one has to provide the strip with overlapping edges. This, in turn, produces a rather undesirable step which contributes to the surface roughness in the surface of the foam. Also, the foaming material shoud not adhere to the inner surface of the strip as it is formed around the inner tube, and in order to avoid this adhesion it is necessary to spray the strip prior to its being folded into a split tube by means of a foam rejecting material. Unfortunately this particularly foam-rejecting material will interfere with the foaming process and will, in fact, directly produce a rough surface.
The latter method has been the subject of an improvement in accordance with German printed patent application 1,704,748. In accordance with this particular patent, it has been proposed to use a foil as a ribbon or strip that is formed around the inner tubing and contains the foam, which foil remains after the process. As the assembly leaves the foam providing machine an outer jacket is established in order to provide the requisite strength. However, it was found that this particular method is not suitable for making flexible thermally insulated tubing because there is no connection or joint between the foam, the foil, and the outer jacket so that, upon flexing and bending of the tubular assembly, the foil as well as the outer wall or stratum of the foam may tear.